[wasm] Fix exception handling with empty try blocks.

R=clemensh@chromium.org
TEST=mjsunit/wasm/exceptions
BUG=v8:8091

Change-Id: I866430e1f3d3e7b93001a4375a869f19de75e65e
Reviewed-on: https://chromium-review.googlesource.com/1203954Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55605}

src/wasm/function-body-decoder.cc

@@ -433,48 +433,37 @@ class WasmGraphBuildingInterface {
     SsaEnv* catch_env = block->try_info->catch_env;
     SetEnv(catch_env);
 
-    TFNode* compare_i32 = nullptr;
-    if (exception == nullptr) {
-      // Catch not applicable, no possible throws in the try
-      // block. Create dummy code so that body of catch still
-      // compiles. Note: This only happens because the current
-      // implementation only builds a landing pad if some node in the
-      // try block can (possibly) throw.
-      //
-      // TODO(kschimpf): Always generate a landing pad for a try block.
-      compare_i32 = BUILD(Int32Constant, 0);
-    } else {
+    // The catch block is unreachable if no possible throws in the try block
+    // exist. We only build a landing pad if some node in the try block can
+    // (possibly) throw. Otherwise the below catch environments remain empty.
+    DCHECK_EQ(exception != nullptr, ssa_env_->go());
+
+    TFNode* if_catch = nullptr;
+    TFNode* if_no_catch = nullptr;
+    if (exception != nullptr) {
       // Get the exception and see if wanted exception.
       TFNode* caught_tag = BUILD(GetExceptionRuntimeId, exception);
       TFNode* exception_tag = BUILD(ConvertExceptionTagToRuntimeId, imm.index);
-      compare_i32 = BUILD(Binop, kExprI32Eq, caught_tag, exception_tag);
+      TFNode* compare_i32 = BUILD(Binop, kExprI32Eq, caught_tag, exception_tag);
+      BUILD(BranchNoHint, compare_i32, &if_catch, &if_no_catch);
     }
 
-    TFNode* if_catch = nullptr;
-    TFNode* if_no_catch = nullptr;
-    BUILD(BranchNoHint, compare_i32, &if_catch, &if_no_catch);
-
     SsaEnv* if_no_catch_env = Split(decoder, ssa_env_);
     if_no_catch_env->control = if_no_catch;
     SsaEnv* if_catch_env = Steal(decoder->zone(), ssa_env_);
     if_catch_env->control = if_catch;
 
-    // TODO(kschimpf): Generalize to allow more catches. Will force
-    // moving no_catch code to END opcode.
     SetEnv(if_no_catch_env);
-    BUILD(Rethrow, exception);
-    Unreachable(decoder);
-    EndControl(decoder, block);
+    if (exception != nullptr) {
+      // TODO(kschimpf): Generalize to allow more catches. Will force
+      // moving no_catch code to END opcode.
+      BUILD(Rethrow, exception);
+      Unreachable(decoder);
+      EndControl(decoder, block);
+    }
 
     SetEnv(if_catch_env);
-
-    if (exception == nullptr) {
-      // No caught value, make up filler nodes so that catch block still
-      // compiles.
-      for (Value& value : values) {
-        value.node = DefaultValue(value.type);
-      }
-    } else {
+    if (exception != nullptr) {
       // TODO(kschimpf): Can't use BUILD() here, GetExceptionValues() returns
       // TFNode** rather than TFNode*. Fix to add landing pads.
       TFNode** caught_values =

test/mjsunit/wasm/exceptions.js

@@ -61,6 +61,21 @@ function assertWasmThrows(runtime_id, values, code) {
   assertWasmThrows(except, [], () => instance.exports.throw_if_param_not_zero(-1));
 })();
 
+// Test that empty try/catch blocks work.
+(function TestCatchEmptyBlocks() {
+  let builder = new WasmModuleBuilder();
+  let except = builder.addException(kSig_v_v);
+  builder.addFunction("catch_empty_try", kSig_v_v)
+      .addBody([
+        kExprTry, kWasmStmt,
+        kExprCatch, except,
+        kExprEnd,
+      ]).exportFunc();
+  let instance = builder.instantiate();
+
+  assertDoesNotThrow(instance.exports.catch_empty_try);
+})();
+
 // Now that we know throwing works, we test catching the exceptions we raise.
 (function TestCatchSimple() {
   let builder = new WasmModuleBuilder();